News:

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this thread began with a question about the vacuum advance pod for the 480 distributor. Reading the thread got me interested in following up and rebuilding my 480 distributor, so I bought a couple of the autozone PN's mentioed, and ordered a GM part (but haven't got it yet). When I disassembled my distributor, I think it had the original vacuum module on it (see photo below); can someone confirm this is the original? or at least the original PN?

... after removal, I compared the spring strength to the new autozone part, and find that the original spring tension is roughly 3-4 X more than the new autozone part... which concerns me. Is this normal? Wouldn't it require a lot more vacuum to affect a timing change wiht a high spring pressure? What gives? IS the autozone part I have too 'different'? Or is there some kind of failure of the original part that could cause this? Yes, I'm totally confused. There appears to be a slight bend in the original; could this cause the stronger spring/diaphragm pressure?

I don't have the Delco cross-reference for the "201 15" VAC unit to determine if it's the one originally installed on the 480 distributor - maybe Jerry does.

The diaphragm housing is probably bent from using it to rotate the distributor while setting timing.

It doesn't matter where you buy a "B28" VAC unit - they're all the same, made by Standard Products (who bought that business from Echlin, Borg-Warner, and Dana) and re-boxed for retail distribution, including through GM (#88924985) and AC-Delco (#D1312C).

Just check it with a MityVac - it should start to move at 4" Hg., and be fully-deployed at 8" Hg.; if not, take it back and get another one. I bought a bunch of them about six months ago from GMPartsDirect.com ($6.00 each), and all of them checked out fine with a MityVac.

The only issue with the B28s is they will only pull 10 degrees .You will have to elongate the slot for the 15 needed .The early deploy is wonderful,No idle fluctuations on low vac. cams .Perfect for Z28 and L/78s .

Steve: I think I understand that the B28 would deploy more quickly/easily based on the light spring pressure. Bye elongation of the 'slot'; are you referring to the slot in the vac pod plate that the end of the rod extends thru? elongate it on the high vacuum end-of-motion I assume...?

Correct,the lighter spring inside the vac.can .And elongate on the high vac. side of the slot .It is best to check the range of advance the can gives with a timing tape, and a Mighty Vac pump instead of relying on the Vac. supplied by the engine .

The only issue with the B28s is they will only pull 10 degrees .You will have to elongate the slot for the 15 needed .The early deploy is wonderful,No idle fluctuations on low vac. cams .Perfect for Z28 and L/78s .

That hasn't been my experience - I've used lots of them on Corvettes in the last year (and I've had one on my Z/28 for eight years), and all of them, fully-deployed, add between 14-17 degrees at 8" Hg. During the consolidation from Echlin/B-W/Dana to Standard Products, some were apparently mis-identified and/or mis-boxed. They work particularly well with full manifold vacuum on '66 L-72 and '67-'69 L-71 3x2 Corvettes, which had "ported" vacuum from the factory.

I must second JohnZ's experience with the B28 vacuum advance can. I recently switched from a B1 to a B28, and it pulls between 17 - 19 degrees advance at idle with my 10 - 11" Hg. idle vacuum. I had to limit it to 16 degrees with an adjustable vacuum advance limiter cam. The B28 is rated to yield 8 distributor (16 crank) degrees at 5.75 – 8” Hg, but I am sure they can vary.

L78 Steve...Is it possible that something is hanging up the vacuum plate? Maybe your the B28 is not functioning properly.

Last night I made the following adjustment to my Holley carb and road tested the car today. See my base line listed earlier in this discussion topic.

Primary side - changed the jets from #68 to #72, and retained the #65 power valve.Secondary side - retained the #76 jets and replaced the power valve block off plug with a #65 power valve.Reinstalled the vacuum secondary check ball.Readjusted the idle mixture screws for a max vacuum between 10 - 11” Hg.

Results: Great throttle response and a noticeable increase in power thru the RPM range to red line! Making the modifications (from the stock set-up) to connect the vacuum advance to full manifold vacuum and pull in the vac advance at idle has really made a difference. These modification really help to make the 302 come alive! The only issue now is after I drive and park the car, I smell a hit of gas so I must be a bit on the rich side. Does anyone else get this smell? I may try #70 jets on the primary side next.

Attached is a picture showing the simple swap between the A.I.R. hose and vacuum advance hose lines (as JohnZ recommended). The vacuum advance is now connected to a full vacuum signal from the t-fitting at the base of the carb, and the A.I.R. hose is connected to the ported source on the carb. The A.I.R. line was blocked with a ¼” section of a plastic golf tee so it actually now a vacuum plug. The lines are easily swapped if necessary for a car show.

I still need to dial in the centrifugal advance curve and I should be all set.

Also, dial back lights are not accurate ,Timing tape is the way to go if you don't have a Sun machine handy .

That may be so with some dial-back lights, but not all. I use an Actron CP-7519 ($70.00), and it indicates EXACTLY the same as my neighbor's $400 Snap-On dial-back light (just doesn't have the digital tach feature, but I use my tach/dwell meter for that anyway).

The dial backs I've tried ,one was off 4 degrees and the other bounced around at around 4000 RPM .I thought it may have been the chain or the cam moving but when I installed the tape with the light set on zero the timing was steady at all speeds .I have a Actron and a Mac .I have a B28 in a 69Z that I had to open up to get the 15 degrees and one on a L/78 that I have not checked .I'll try it and see what it gives .

Also, dial back lights are not accurate ,Timing tape is the way to go if you don't have a Sun machine handy .

That may be so with some dial-back lights, but not all. I use an Actron CP-7519 ($70.00), and it indicates EXACTLY the same as my neighbor's $400 Sun dial-back light (just doesn't have the digital tach feature, but I use my tach/dwell meter for that anyway).

Yup, I have an old Snap-On MT241A timing light and it has been flawless!